Underground physics: Searching for
neutrinos in deep places

by Dave Jacqué

A new physics experiment combines thousands of tons of steel plates, a powerful particle accelerator and 450 miles of solid rock to reveal the secrets of a particle that can be very hard to detect and study. Construction was completed in early 2005, and the experiment has now begun its initial five-year run.

The experiment, dubbed the Main Injector Neutrino Oscillation Search (MINOS), was built by a collaboration of more than 30 national laboratories, universities and scientific institutions from six nations. Fermi National Accelerator Laboratory (Fermilab) has the lead role.

Argonne scientists and engineers were instrumental in getting the experiment launched in the late 1980s and early 1990s, and they later designed and built many of the detector components. This work included setting up “factories” to build the plastic scintillator detector modules at Caltech, the University of Minnesota and Argonne; building much of the “front-end” electronics that receive and record signals from the detectors; and installing the detectors.

As the name implies, neutrinos have no electrical charge. They are produced in vast numbers in some nuclear reactions, such as those that occur in the fusion processes that light the stars, in the nuclear power plants that light our cities and those that occurred in the Big Bang. But neutrinos are the introverts of subatomic particle society: they rarely interact with other matter. Neutrinos produced by the sun can, and routinely do, pass entirely through the entire planet Earth without interacting with a single atom. Billions of neutrinos produced in the core of the sun are passing through your body as you read this sentence — even at night.

There are three kinds or flavors of neutrinos: electron, muon and tau. The heaviest neutrino could weigh as little as one ten-millionth of the mass of an electron.

There is another catch: neutrinos are constantly changing from one type to another and back again. Every neutrino is actually a quantum-mechanical blend of the three flavors. Over time, the quantum waves that accompany the different flavors get out of step, and an electron neutrino morphs into a muon neutrino or a tau neutrino and back again. This “oscillation” provides the best evidence that the particles actually have distinct, non-zero masses.

But the very properties that make neutrinos interesting to physicists make them notoriously difficult to study. If they can pass through the entire Earth without interacting with anything, how do you “catch” them?

The MINOS experiment aims to overcome this difficulty by creating lots of neutrinos, aiming them at a big detector and putting lots of distance between the source and detector to give the neutrinos a chance to oscillate.

A specially built beamline at Fermilab produces a beam of nearly pure muon neutrinos that passes through the MINOS Near Detector, located at Fermilab, before it travels to the Far Detector in Minnesota. Some of the particles change to tau neutrinos during the trip, and a few are recorded by the detectors. The data may lead to discoveries about the morphing mechanism, better estimates of the mass of each type of neutrino and much more.

“ This will be the first long-baseline neutrino experiment done under controlled conditions with high intensity, so we can actually measure these oscillation parameters precisely,” said David Ayres, who leads Argonne's team of physicists and engineers working on MINOS.

NuMI

Neutrino production begins when protons pick up energy as they circulate around the Main Injector. A beam of protons, with each particle packing 120 billion electron volts of energy, is extracted from the accelerator and aimed at a graphite target.

“ This will be an extremely intense beam, even by Fermilab standards, and we are taking very stringent precautions to contain and monitor the radioactive material that it produces as a byproduct of the process of creating neutrinos.” Ayres said.

The proton beam, a millimeter across, will interact with carbon atoms in a target — a stack of graphite rectangles about three feet long — to produce a shower of secondary particles. The beam of secondary particles includes mesons, a family of particles consisting of a quark and an anti-quark. The beam is focused by two magnetic “horns,” trumpet-shaped metal devices driven by 200,000-amp pulses of electric current. The distance between the target and horns can be changed like a zoom lens, so that the beam of charged particles created in the target can be “focused.” The design allows the energy of the meson beam (and the resulting neutrino beam) to be changed easily during the experiment.

The secondary particles enter a pipe more than 2,000 feet long, where they decay into trillions of neutrinos and other particles. The decay pipe ends abruptly in a hadron absorber. This is a stack of water-cooled aluminum and steel blocks and concrete that stop leftover protons and mesons. Behind that lies 240 meters of unexcavated bedrock, which absorbs muons. The vast majority of neutrinos blithely pass through these minor obstacles.

Behind the muon absorber is a large cavity 300 feet below the surface containing the MINOS Near Detector, a small-scale version of the MINOS Far Detector. Its 980 tons of steel and scintillator provide a “reference” for unoscillated neutrinos emerging from the NuMI beam. After it passes through the near detector, the neutrino beam is about six feet in diameter, heading north-northwest at a 3.3-degree angle down into Earth. The neutrino beam sees nothing but solid rock for another 450 miles and 0.0025 seconds.